A mucous cyst (sometimes referred to as a digital mucous cyst) is a common but benign nodule that develops on the fingers or toes, particularly near the distal joints.

These small fluid-filled sacs originate from an interphalangeal joint of a finger or toe. (Specifically, they usually originate from the DIP joint or end joint of the finger or toe.) They may also occur at the middle joint or PIP joint, although this is less common. Often, the cysts are painless. While their exact cause is unknown, a mucous cyst is most commonly associated with an osteoarthritis (OA) diagnosis.

The Anatomy of the Cyst

  • A mucous cyst is filled with joint fluid and is connected by a small stalk to the joint the cyst is growing from.
  • Often there is a small spur in the affected joint as well. This may produce a weakening of the capsule (or lining) of the joint, allowing fluid to escape and form a mucous cyst. 
  • Typically a mucous cyst will appear slightly to the side of the dorsum (top) or back of the finger/toe. 
  • The capsule in the midportion of the joint is typically thickened by the extensor tendon. 
  • The joint may also be enlarged secondary to bony spurs related to an arthritis diagnosis.

The Symptoms

In many cases, a mucous cyst itself is asymptomatic. Discomfort, however, may originate from the presence of the cyst and/or an accompanying arthritis diagnosis. 

Large cysts are in danger of rupturing due to thinning of the skin overlying the cyst. Since the cyst communicates with the joint (i.e. is connected), there is also a risk of joint infection, which can threaten the finger.

The Treatment

We advise against aspirating, poking, or draining a cyst due to the risk of infection (and since the rate of recurrence in these cases is high). Instead, small asymptomatic cysts are usually observed. 

A large cyst that is painful, or which has made the overlying skin dangerously thin, is usually removed surgically. The surgery is usually performed on an outpatient basis with local standby or twilight anesthesia. A small incision is made over the cyst and the skin is carefully peeled back, exposing the cyst. It is traced down to the joint and removed. 

If a spur is present, that portion of the joint is opened (arthrotomy) and the spur is also removed to reduce the rate of recurrence. 

A small dressing is applied afterward in both scenarios. 

Risks with this surgery are anesthesia-related risks, problems related to wound-healing or infection, and potential recurrence of the cyst. Recurrence occurs in approximately 3% of cases.